Have you checked the Tomcat logs? Did you get rid of the examples that ship with Tomcat or did you add your servlet to the existing "examples" app? If you added it, can you still run the other examples? If not, have you checked the Tomcat logs to see if your addition blew up the app?
I believe that in Servlet Spec 2.3 compliant apps (which Tomcat 4.1 is) the order of elements in the web.xml file is critical. If you didn't enter them in the correct order, you should see some Sax parse errors in the logs.
This sounds like more of a Tomcat issue than a servlet one. Moving.